Receiver and receiving method for hierarchical modulation in single frequency networks

ABSTRACT

A receiver recovers local service data symbols from first Orthogonal Frequency Division Multiplexed (OFDM) symbols in the presence of second OFDM symbols, the second OFDM symbols carry national broadcast data symbols and modulated on to the sub-carriers of the second OFDM symbols using a first modulation scheme, and the first OFDM symbols carry the national broadcast data symbols and the local service data symbols from a local insertion pipe and modulated on to the sub-carriers of the first OFDM symbols using a second modulation scheme. The receiver comprises an OFDM detector which includes an equalizer for recovering local service modulated sub-carriers of the second modulation scheme by generating an estimate of a combined channel ([H n (z)+H l (z)]) via which the first and second OFDM symbols have passed using the pilot sub-carrier symbols of the first and second OFDM symbols; generating an estimate of national broadcast modulation symbols from the modulated data bearing sub-carriers of the first modulation scheme from the second OFDM symbols (Ŝ(z)); generating an estimate of a convolution of the combined channel and the national broadcast modulation symbols (Ŝ(z)[H n (z)+H l (z)]); generating an estimate of a component of the received base band signal representing the local service modulation symbols of the first OFDM symbol; generating an estimate of a channel via which the first OFDM symbols were received using the local pilot symbols (Ĥ l (z)); and generating an estimate of local service data symbols from a combination of the estimate of the component of the received signal representing the modulation symbols carrying the local service data and the estimate of the channel via which the first OFDM symbols were received 
     
       
         
           
             
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CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/822,455, filed Aug. 10, 2015, which is a continuation ofU.S. patent application Ser. No. 13/747,531, filed Jan. 23, 2013, whichis a continuation-in-part of International Application No.PCT/GB2011/051964 filed Oct. 12, 2011, which claims foreign priority toApplication Nos. GB 1017566.9 and GB 1100736.6 filed Oct. 18, 2010 andJan. 17, 2011, respectively, a continuation-in-part of InternationalApplication No. PCT/GB2011/051843 filed Sep. 29, 2011, which claimsforeign priority to Application No. GB1017565.1 filed Oct. 18, 2010, anda continuation-in-part of International Application No.PCT/GB2011/051777 filed Sep. 21, 2011, which claims foreign priority toApplication No. GB 1017567.7 filed Oct. 18, 2010. The content of each ofthe foregoing applications is hereby incorporated by reference into thepresent application in its entirety.

FIELD OF INVENTION

The present invention relates to receivers for receiving data viaOrthogonal Frequency Division Multiplexed (OFDM) symbols in which thedata is provided from a plurality of different data pipes. The presentinvention also relates to transmitters for transmitting data viaOrthogonal Frequency Division Multiplexed (OFDM) symbols in which thedata is provided from a plurality of different data pipes.

Embodiments of the present invention find application in receiving datacommunicated using OFDM symbols which are transmitted usingcommunication systems which comprise a plurality of base stationsdisposed throughout a geographical area. In some embodiments thecommunication system is arranged to broadcast video, audio or data.

BACKGROUND OF THE INVENTION

Orthogonal Frequency Division Multiplexing (OFDM) is a modulationtechnique which has found much favour in communication systems, such asfor example those designed to operate in accordance with the first andsecond generation Digital Video Broadcasting terrestrial standards(DVB-T/T2) and is also being proposed for fourth generation mobilecommunication systems which are also known as Long Term Evolution (LTE).OFDM can be generally described as providing K narrow band sub-carriers(where K is an integer) which are modulated in parallel, eachsub-carrier communicating a modulated data symbol such as QuadratureAmplitude Modulated (QAM) modulation symbol or Quaternary Phase-shiftKeying (QPSK) modulation symbol. The modulation of the sub-carriers isformed in the frequency domain and transformed into the time domain fortransmission. Since the data symbols are communicated in parallel on thesub-carriers, the same modulated symbols may be communicated on eachsub-carrier for an extended period, which can be longer than thecoherence time of the radio channel. The sub-carriers are modulated inparallel contemporaneously, so that in combination the modulatedcarriers form an OFDM symbol. The OFDM symbol therefore comprises aplurality of sub-carriers each of which has been modulatedcontemporaneously with a different modulation symbol.

In the Next Generation for Hand held (NGH) television system it has beenproposed to use OFDM to transmit television signals from base stationsdisposed throughout a geographical area. In some examples the NGH systemwill form a network in which a plurality of base stations communicateOFDM symbols contemporaneously on the same carrier frequency therebyforming a so-called single frequency network. As a result of some of theproperties of OFDM, a receiver may receive the OFDM signals from two ormore different base stations which can then be combined in the receiverto improve the integrity of the communicated data.

Whilst a single frequency network has advantages in terms of operationand improved integrity of the communicated data, it also suffers adisadvantage if data local to a part of the geographical area isrequired to be communicated. For example, it is well known in the UnitedKingdom that the national carrier, the BBC, broadcasts television newsthroughout the entire national network but then switches, at certaintimes, to “local news” in which a local news programme is transmittedwhich is specifically related to a local area within the nationalnetwork. However, the United Kingdom operates a multi-frequency DVB-Tsystem so that the insertion of local news or local content of any sortis a trivial matter because the different regions transmit DVB-Ttelevision signals on different frequencies and so television receiverssimply tune to an appropriate carrier frequency for the region withoutinterference from other regions. However, providing an arrangement toinsert data locally in a single frequency network presents a technicalproblem.

A known technique for providing a hierarchical or multi-layer modulationscheme in a single frequency OFDM network is disclosed in US2008/0159186. The hierarchical modulation scheme provides a plurality ofmodulation layers which can be used to communicate data from differentdata sources or pipes contemporaneously.

SUMMARY OF INVENTION

According to the present invention there is provided a receiver forreceiving and recovering local service data symbols from firstOrthogonal Frequency Division Multiplexed (OFDM) symbols in the presenceof second OFDM symbols. The first and the second OFDM symbols include aplurality of sub-carrier symbols formed in the frequency domain, thesecond OFDM symbols carrying national broadcast data symbols andmodulated on to the sub-carriers of the second OFDM symbols using afirst modulation scheme to form national broadcast modulation symbols,and the first OFDM symbols carrying the national broadcast data symbolsand the local service data symbols and modulated on to the sub-carriersof the first OFDM symbols using a second modulation scheme. The firstand the second OFDM symbols both include the same pilot sub-carriersymbols and the first OFDM symbols include local pilot symbols. Thereceiver comprises a tuner which is arranged in operation to detect aradio frequency signal representing a combination of the first and thesecond OFDM symbols and to form a received base band signal representingthe combined first and second OFDM symbols, an OFDM detector which isarranged in operation to recover modulation symbols carrying the localservice data symbols from the data bearing sub-carriers of the firstOFDM symbols, and a de-modulator arranged in operation to generate anestimate of the local service data symbols from the modulation symbolscarrying the local service data symbols. The OFDM detector includes anequaliser for recovering the local service data symbols of the secondmodulation scheme by

generating an estimate of a combined channel ([H_(n)(z)+H_(l)(z)]) viawhich the first and second OFDM symbols have passed using the pilotsub-carrier symbols of the first and second OFDM symbols;

generating an estimate of the national broadcast modulation symbols fromthe modulated data bearing sub-carriers of the first modulation schemefrom the second OFDM symbols (Ŝ(z));

generating an estimate of a convolution of the combined channel and thenational broadcast modulation symbols (Ŝ(z)[H_(n)(z)+H_(l)(z)]);

generating an estimate of a component of the received base band signalrepresenting the local service modulation symbols of the first OFDMsymbols by subtracting from the received signal the generated estimateof the national broadcast modulation symbols convolved with the estimateof the combined channel to form an intermediate result(D(z)H_(l)(z)≈R(z)−Ŝ(z)[H_(n)(z)+H_(l)(z)]);

generating an estimate of a channel via which the first OFDM symbolswere received using the local pilot symbols (Ĥ_(l)(z)), and

generating an estimate of local service data symbols from a combinationof the estimate of the component of the received signal representing themodulation symbols carrying the local service data and the estimate ofthe channel via which the first OFDM symbols were received

$\left( {{\overset{\sim}{D}(z)} \approx \frac{{R(z)} - {{\hat{S}(z)}\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}}{{\hat{H}}_{l}(z)}} \right).$

According to the arrangement disclosed in US 2008/0159186 published 3Jul. 2008, a single carrier frequency OFDM network is provided with afacility for communicating data from different pipes contemporaneouslyby using two related modulations schemes to form a plurality ofdifferent modulation “layers”. As will be explained shortly, a firstmodulation scheme is selected for communicating data from a first datapipe and a second modulation scheme related to the first modulationscheme is selected for communicating data according to the first and asecond communications pipes. The second modulation scheme comprises anincreased number of constellation points in the complex plane than thefirst modulation scheme. The data from the first pipe maybe from anational broadcast, whereas the data from the second pipe is from alocal broadcast signal, which is directed to an area which is a subsetof the area of the national broadcast signal.

According to example embodiments of the present invention, a receiver isarranged to recover data symbols according to the local service using anequaliser, which can compute an estimate of the local service modulationsymbols from OFDM symbols which are carrying both the local service andthe national broadcast service carried using data sub-carriers modulatedwith the second modulation scheme in the presence of OFDM symbols whichare only carrying the national broadcast data symbols modulated with thefirst modulation scheme. This is achieved by making a first coarseestimate of the first modulation symbols and then convolving thisestimate with an estimate of the channel through which the first andsecond OFDM symbols have passed. The estimate of the channel is madeusing the pilot sub-carriers as these coincide in the first and thesecond OFDM symbols. Subtracting the result of the convolution from thereceived signal and dividing by the estimate of the channel throughwhich only the second OFDM symbol has passed provides an estimate of themodulation symbols according to the second modulation scheme, which arecarrying the local service data symbols. The channel through which onlythe second OFDM symbol has passed can be estimated using the localservice insertion pilots carried on known sub-carriers of the secondOFDM symbol.

The receiver can be therefore arranged to detect and recover data fromOFDM symbols communicated by a communication system which is arrangedsuch that one or more base stations from a plurality of base stationswhich form a communications network are selected to transmit localcontent via OFDM symbols which have sub-carriers modulated in accordancewith the second modulation scheme. This is because the first modulationscheme forms a sub-set of constellation points in the complex plane ofthe second modulation scheme, which can be thought of as a more coarseversion of the second modulation scheme, so that differentiation betweenconstellation points of the first modulation symbols in the complexplane allows the data from the national broadcast signal to be moreeasily recovered. Furthermore, because other base stations may not becommunicating the local insertion pipe data, the receiver, within thegeographical area in which these other base stations are disposed, willstill be able to detect the data from the national broadcast signal.Accordingly, an effective and efficient way of inserting local contentin a single frequency network is provided.

In some examples the OFDM detector includes an equaliser which isarranged in operation to generate the estimate of the local service datasymbols from the combination of the estimate of the component of thereceived base band signal representing the modulation symbols carryingthe local service data symbols and the estimate of the channel via whichthe first OFDM symbols were received by dividing the estimate of thecomponent of the received signal representing the local service datasymbols by the estimate of the local channel. An estimate of each of themodulation symbols carrying the local service data symbols from thefirst OFDM symbol is thereby recovered, and by de-mapping the modulationsymbols carrying the local service data symbols the estimate of thelocal service data symbols is generated. However, whilst this provides asimple and effective equalisation technique for at least reducing orcancelling the effects of the channel, in a multi-path fading channel,frequency nulls can be produced in the channel, which can result innoise amplification or cause a modulation symbol to produce an amplifiedvalue which is equal to a maximum possible real and imaginary samplecomponents, thereby losing data which such modulation symbols carry.

In other examples, the equaliser includes a local serviceequaliser/demapper which is arranged in operation to generate theestimate of the local service data symbols from the combination of theestimate of the component of the received base band signal representingthe modulation symbols carrying the local service modulation symbols andthe estimate of the channel via which the first OFDM symbols werereceived. This is achieved by calculating a log likelihood ratio foreach of the local service data symbols from the estimate of thecomponent of the received signal representing the modulation symbolscarrying the local service data symbols and the estimate of the localchannel, and estimating the local service data symbols from the loglikelihood ratio calculations. As such, by using a log likelihood ratiocalculation for detecting the local service data symbols from the firstOFDM symbols and the estimate of the local service channel, no divisionby the channel occurs in the frequency domain. Accordingly, the localservice data symbols as well as the national broadcast data symbols canbe recovered in a multi-path fading channel.

Correspondingly in other example embodiments the OFDM detector alsoincludes an equaliser/demapper, which calculates a log likelihood ratiofor the national broadcast data symbols from the components of the firstand second OFDM representing the national broadcast data symbols and thecombined channel estimate through which the symbols were received.

In some examples, the equaliser can be arranged to re-generate anestimate of the national broadcast modulation symbols, by re-generatingan estimate of the component of the received base band signalrepresenting modulation symbols of the first OFDM symbol carrying localservice data symbols by combining the estimate of the modulation symbolsrepresenting the local service data symbols with the channel via whichthe first OFDM symbols were received, generating an estimate of acomponent of the received base band signal representing the nationalbroadcast modulation symbols by subtracting the re-generated estimate ofthe component of the received base band signal representing themodulation symbols carrying the local service data symbols from thereceived base band signal, and dividing by the combined channel.Furthermore, the re-generated estimate of the national broadcastmodulation symbols may be used to make a refined estimate of the localservice modulation symbols. The equaliser can therefore be arranged togenerate a refined estimate of the local service data symbols from thelocal service modulation symbols. Thus in a form of turbo detection, there-generated estimate of the national broadcast modulation symbols canbe used to generate a further refined estimate of the local servicemodulation symbols and the process of detection further repeated togenerate further refined estimates.

According to the present invention there is provided a transmitter forcommunicating data using Orthogonal Frequency Division Multiplexed(OFDM) symbols, the OFDM symbols including a plurality of sub-carriersymbols formed in the frequency domain for modulating with the data tobe carried and pilot sub-carriers. The transmitter includes a modulatorarranged in operation to receive on a first input, national broadcastdata symbols from a first data pipe according to a national broadcastchannel for transmission, to receive on a second input, local servicedata symbols from a local service insertion data pipe according to alocal service channel for transmission, and to modulate the data bearingsub-carrier signals of the OFDM symbols with either the nationalbroadcast data symbols or the local service data symbols and thenational broadcast symbols. The modulation of the data bearingsub-carrier signals of the OFDM symbols with the national broadcast datasymbols is performed by mapping the data symbols according to a firstmodulation scheme, and the modulation of the sub-carrier signals of theOFDM symbols with the local service data symbols and the nationalbroadcast data symbols is performed by mapping the national broadcastdata symbols and the local service data symbols according to a secondmodulation scheme. The modulator modulates the pilot sub-carriers of theOFDM symbols with pilot symbols to form the OFDM symbols fortransmission. The first modulation scheme is a lower order modulationscheme providing first modulation symbols with values from a smallernumber of constellation points in the complex plane than the secondmodulation scheme which is a higher order modulation scheme, the secondmodulation scheme providing second modulation symbols with values whichare disposed in the complex plane about corresponding values of thefirst modulation scheme, with the effect that detection of one of thesecond modulation symbols of the second modulation scheme will providethe local insertion data symbols and the national broadcast data symbolsand allow detection of national broadcast modulation symbols from thefirst modulation scheme providing the national broadcast data symbols,in the presence of modulation symbols from the second modulation scheme.If the modulator is arranged to modulate the data bearing sub-carrierswith both the local service data symbols and the national broadcastsymbols, to include local pilot symbols with the local service datasymbols.

Embodiments of the present invention are arranged to include local pilotsymbols which with local service data symbols when communicated withnational broadcast data symbols which are communicated using modulationsymbols of a second modulation scheme whereas when transmitting thenational broadcasting data symbols alone a first modulation scheme isused, which is related to the second modulation scheme in a hierarchicalmanner. The local pilot symbols can be used by a receiver to estimate achannel via which the local service modulation symbols have passed,which can allow the local service data symbols modulated using thesecond modulation scheme to be detected in the presence of OFDM symbolsmodulated with the first modulation scheme. For example, a receiver canbe arranged to recover data symbols according to the local service usingan equaliser, which can estimate the local service modulation symbolsfrom OFDM symbols which are carrying both the local service and thenational broadcast service carried using data sub-carriers modulatedwith the second modulation scheme in the presence of OFDM symbols whichare only carrying the national broadcast data symbols modulated with thefirst modulation scheme. This is achieved by making a first coarseestimate of the first modulation symbols and then convolving thisestimate with the channel estimate using the pilot sub-carriers.Subtracting the result from the received signal and dividing by anestimate of the channel of the first OFDM symbols provides an estimateof the local service modulation symbols according to the secondmodulation scheme.

The receiver can be therefore arranged to detect and recover data fromOFDM symbols communicated by a communication system which is arrangedsuch that one or more base stations from a plurality of base stationswhich form a communications network are selected to transmit localcontent via OFDM symbols which have sub-carriers modulated in accordancewith the second modulation scheme. Thus, the second modulation scheme isused to convey data symbols from both the first data pipe and the localinsertion pipe. Because of the arrangement of the second modulationscheme with respect to the first modulation scheme, the data symbolsfrom the first data pipe may be received even when transmitted on thesame radio frequency carrier, because detection of a constellation pointfrom the first modulation scheme will require a lower signal to noiseratio than the second modulation scheme. This is because the firstmodulation scheme forms a sub-set of constellation points in the complexplane of the second modulation scheme, which can be thought of as a morecoarse version of the second modulation scheme, so that differentiationbetween constellation points of the first modulation symbols in thecomplex plane allows the data from the first data pipe to be more easilyrecovered. Furthermore, because other base stations may not becommunicating the local insertion pipe data, the receiver, within thegeographical area in which these other base stations are disposed, willstill be able to detect the data from the first data pipe. This isbecause OFDM signals transmitted from a neighbouring base station on thecommon radio frequency carrier signal using the second modulation schemewill simply appear as noise with respect to a detector detecting OFDMsymbols according to the first modulation scheme. Accordingly, aneffective and efficient way of inserting local content in a singlefrequency network is provided.

The local pilots can be inserted by either pre-allocating there locationwithin the data or puncturing the local data, which replaces localservice data symbols with local pilot symbols.

According to the present invention there is provided a transmitter forcommunicating data using Orthogonal Frequency Division Multiplexed(OFDM) symbols, the OFDM symbols including a plurality of sub-carriersymbols formed in the frequency domain for modulating with the data tobe carried and pilot sub-carriers and the transmitter is arranged toform a Multiple Input Multiple Output (MIMO) scheme. The transmittercomprises a modulator arranged in operation to receive on a first input,national broadcast data symbols from a first data pipe according to anational broadcast channel for transmission, to receive on a secondinput, local service data symbols from a local service insertion datapipe according to a local service channel for transmission, and tomodulate the data bearing sub-carrier signals of the OFDM symbols witheither the national broadcast data symbols or the local service datasymbols and the national broadcast symbols. The modulation of the databearing sub-carrier signals of the OFDM symbols with the nationalbroadcast data symbols is performed by mapping the data symbolsaccording to a first modulation scheme, and the modulation of thesub-carrier signals of the OFDM symbols with the local service datasymbols and the national broadcast data symbols is performed by mappingthe national broadcast data symbols and the local service data symbolsaccording to a second modulation scheme. A MIMO encoder is arranged toreceive the OFDM symbols and to form at least first and second versionsof the OFDM symbols, a frequency interleaver arranged to receive thefirst and second versions of the OFDM symbols and to interleave theposition of the respective modulated sub-carriers; and a pilot signalinserter arranged to receive the first and second version of the OFDMsymbols and to insert first pilot symbols at the pilot sub-carrierlocations of the first version of the OFDM symbols and second pilotsymbols at the pilot sub-carrier locations of the second version of theOFDM symbols; and a radio frequency modulator which is arranged tomodulate a radio frequency carrier signal with the first and secondversions of the OFDM symbols for transmission via first and secondantennas respectively. The first modulation scheme is a lower ordermodulation scheme providing first modulation symbols with values from asmaller number of constellation points in the complex plane than thesecond modulation scheme which is a higher order modulation scheme, thesecond modulation scheme providing second modulation symbols with valueswhich are disposed in the complex plane about corresponding values ofthe first modulation scheme, with the effect that detection of one ofthe second modulation symbols of the second modulation scheme willprovide the local insertion data symbols and the national broadcast datasymbols and allow detection of national broadcast modulation symbolsfrom the first modulation scheme providing the national broadcast datasymbols, in the presence of modulation symbols from the secondmodulation scheme. If the modulator is arranged to modulate the databearing sub-carriers with both the local service data symbols and thenational broadcast symbols, the transmitter is arranged to

to generate local pilot symbols at predetermined locations within thefirst and second OFDM symbols,

to de-interleave the local pilot symbols from the locations within thefirst and second OFDM symbols, the de-interleaving being a reversemapping of the interleaving performed by the frequency interleaver, and

to include the local pilot symbols with the local service data symbolsat the locations determined by the de-interleaving.

Embodiments of the present invention are arranged to include local pilotsymbols which with local service data symbols when communicated withnational broadcast data symbols which are communicated using modulationsymbols of a second modulation scheme whereas when transmitting thenational broadcasting data symbols alone a first modulation scheme isused, which is related to the second modulation scheme in a hierarchicalmanner. The local pilot symbols can be used by a receiver to estimate achannel via which the local service modulation symbols have passed,which can allow the local service data symbols modulated using thesecond modulation scheme to be detected in the presence of OFDM symbolsmodulated with the first modulation scheme. However, if the transmitterand a receiver are arranged to form a MIMO scheme which is to be usedwith a conventional transmitter architecture also used for a SISO or aMISO scheme then an additional technical problem is created becausepilot sub-carrier symbols are conventionally introduced after frequencyinterleaving has been performed. However the local pilots must beinserted with the local service data as part of the second modulationscheme, which must be introduced before the frequency interleaver.Accordingly embodiments of the present invention provide a transmitterwith a frequency de-interleaver which receives a representation of alocation of the local pilot symbols to be introduced into the OFDMsymbols and de-interleaves those locations so that when the OFDM symbolshave passed through the frequency interleaver of the transmitter, thelocal pilot symbols are once again arranged on sub-carriers at theirdesired locations.

A receiver can be therefore arranged to detect and recover data fromOFDM symbols communicated by a communication system which is arrangedsuch that one or more base stations from a plurality of base stationswhich form a communications network are selected to transmit localcontent via OFDM symbols which have sub-carriers modulated in accordancewith the second modulation scheme and which implement a MIMO scheme.Thus, the second modulation scheme is used to convey data symbols fromboth the first data pipe and the local insertion pipe. Because of thearrangement of the second modulation scheme with respect to the firstmodulation scheme, the data symbols from the first data pipe may bereceived even when transmitted on the same radio frequency carrier,because detection of a constellation point from the first modulationscheme will require a lower signal to noise ratio than the secondmodulation scheme. This is because the first modulation scheme forms asub-set of constellation points in the complex plane of the secondmodulation scheme, which can be thought of as a more coarse version ofthe second modulation scheme, so that differentiation betweenconstellation points of the first modulation symbols in the complexplane allows the data from the first data pipe to be more easilyrecovered. Furthermore, because other base stations may not becommunicating the local insertion pipe data, the receiver, within thegeographical area in which these other base stations are disposed, willstill be able to detect the data from the first data pipe. This isbecause OFDM signals transmitted from a neighbouring base station on thecommon radio frequency carrier signal using the second modulation schemewill simply appear as noise with respect to a detector detecting OFDMsymbols according to the first modulation scheme. Accordingly, aneffective and efficient way of inserting local content in a singlefrequency network is provided.

The local pilots can be inserted by either pre-allocating there locationwithin the data or puncturing the local data, which replaces localservice data symbols with local pilot symbols.

Various further aspects and features of the present invention aredefined in the appended claims and include a method of receiving.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings in which likeparts are referred to using the same numerical designations and inwhich:

FIG. 1 is a schematic representation of a plurality of base stationswhich form a single frequency network for broadcasting for example videosignals which may form part of a Next Generation Hand-held (NGH) TVbroadcasting system;

FIG. 2 is a schematic block diagram of an example transmitter accordingto the prior art;

FIG. 3a is a schematic representation of a complex plane providing anillustration of signal constellation points for a first modulationscheme of QPSK; and FIG. 3b is a schematic representation of a complexplane providing an illustration of signal constellation points for asecond modulation scheme of 16QAM according to the prior art;

FIG. 4 is a schematic block diagram of part of a transmitter used in oneor more of the base stations shown in FIG. 1 according to the presenttechnique which supports SISO or MISO;

FIG. 5 is a schematic block diagram of an example modulator which formspart of the transmitter shown in FIG. 4;

FIG. 6 is an illustrative representation of two neighbouring basestations forming two cells A and B which are using a first modulationscheme of 16QAM and a second modulation scheme of 64QAM respectively;

FIG. 7 is a schematic representation showing the effects on theconstellation points as received by a mobile device at three differentpositions X, Y, Z between the two base stations A and B of FIG. 6;

FIG. 8 is an illustrative representation of constellation points in acomplex plane for a first modulation scheme of 16QAM superimposed on asecond modulation scheme of 64QAM;

FIG. 9a is an illustrative representation of a cluster of four cellsserved by four base stations according to the present technique; FIG. 9bis a graphical representation of a plot of frequency with respect totime providing an illustration of a time division multiplexed framestructure; and FIG. 9c is an illustrative representation of a pattern ofcell clusters according to the present technique;

FIG. 10 is an illustrative representation of two neighbouring basestations forming two cells A and B which are using a first modulationscheme of 16QAM and a second modulation scheme of 64QAM respectively,and a mobile receiver which may be arranged to recover local serviceinsertion data in the presence of signals from both the first modulationscheme and the second modulation scheme the signal from cell Btransiting a channel impulse response h_(n)(t) and the signal from cellA transiting a channel impulse response h_(l)(t);

FIG. 11a is a schematic representation of a complex plane providing anillustration of signal constellation points for a first modulationscheme of QPSK; and FIG. 11b is a schematic representation of a complexplane providing an illustration of signal constellation points for asecond modulation scheme of 16QAM wherein reception is without noise andperfect channel estimation;

FIG. 12a is a schematic representation of a complex plane providing anillustration of signal constellation points for a first modulationscheme of QPSK, when received in the presence of the second modulationscheme; but with the signal from each cell transiting through channelsof different channel impulse responses and FIG. 12b provides acorresponding representation of the same signal after equalisation usinga conventional equaliser with perfect channel estimation;

FIG. 13a is a schematic representation of a complex plane providing anillustration of signal constellation points after subtractingS_(est)(z)[(H_(l)(z)+H_(n)(z)] and FIG. 13b is the result of dividingthe signal represented in FIG. 13a by H_(l)(z) assuming perfect channelestimation in which the local service insertion channel H_(l)(z) isknown exactly;

FIG. 14a is an illustrative representation of narrow band carriers of anOFDM symbol carrying the national broadcast signal; FIG. 14b is anillustrative representation of narrow band carriers of an OFDM symbolcarrying both the national signal and the local service insertionsignal; and FIG. 14c is an illustrative representation of narrow bandcarriers of an OFDM symbol carrying the local service insertion signal,but adapted in accordance with the present technique to include localpilots;

FIG. 15 is a schematic block diagram of a transmitter used in one ormore of the base stations according to the present technique, whichsupports MIMO;

FIG. 16 is a graphical plot of bit error rate with respect to signal tonoise ratio for example of a low density parity check (LDPC) coded OFDMtransmitter-receiver chain, with error correction encoding of rate ½, ⅗,⅔ and ¾, a first modulation scheme of 16QAM, a second modulation schemeof 64QAM and in which the receiver is considered to be located withincoverage area of cell A and to receive OFDM symbols with 99% of thesignal power from base station A and 1% from base station B with thesignal from B arriving at the receiver 4.375 us after the signal frombase station A as illustrated by the example diagram shown in FIG. 6;

FIG. 17 is a graphical plot of bit error rate with respect to signal tonoise ratio for the example of a LDPC coded OFDM transmitter-receiverchain, with error correction encoding of rate ½, ⅗, ⅔, and ¾, a firstmodulation scheme of 16QAM, a second modulation scheme of 64QAM and inwhich the receiver is considered to be located within coverage area ofcell A and to receive OFDM symbols with 80% of the signal power frombase station A and 20% from base station B with the signal from Barriving at the receiver 2.2 μs after the signal from base station A asillustrated by the example diagram shown in FIG. 6;

FIG. 18 is a graphical plot of bit error rate with respect to signal tonoise ratio for example of a LDPC coded OFDM transmitter-receiver chain,with error correction encoding of rate ½, ⅗, ⅔ and ¾, a first modulationscheme of 16QAM, a second modulation scheme of 64QAM and in which thereceiver is considered to be located within coverage area of cell A andto receive OFDM symbols with 99% of signal power from base station A and1% from base station B with zero delay between the signal times ofarrival from the two cells illustrated by the example diagram shown inFIG. 6;

FIG. 19 is a graphical plot of bit error rate with respect to signal tonoise ratio for example of a LDPC coded OFDM transmitter-receiver chain,with error correction encoding of rate ½, ⅗, ⅔ and ¾, a first modulationscheme of 16QAM, a second modulation scheme of 64QAM and in which thereceiver is considered to be located within coverage area of cell A andto receive OFDM symbols with 60% of signal power from base station A and40% from base station B with zero delay between the signal times ofarrival from the two cells illustrated by the example diagram shown inFIG. 6;

FIG. 20 is a graphical plot of bit error rate with respect to signal tonoise ratio for example of a LDPC coded OFDM transmitter-receiver chain,with error correction encoding of rate ½, ⅗, ⅔ and ¾, a first modulationscheme of 16QAM, a second modulation scheme of 64QAM and in which thereceiver is considered to be located within coverage area of cell A andto receive OFDM symbols with 50% signal power from base station A and50% from base station B with zero delay between the signal times ofarrival from the two cells illustrated by the example diagram shown inFIG. 6;

FIG. 21 is a graphical plot of bit error rate with respect to signal tonoise ratio for example of a LDPC coded OFDM transmitter-receiver chain,with error correction encoding of rate ½, ⅗, ⅔, a first modulationscheme of 16QAM, a second modulation scheme of 64QAM and in which thereceiver is considered to be located within coverage area of cell B andto receive OFDM symbols with 10% of signal power from base station A and90% from base station B with the signal from A arriving at the receiver2.2 μs after the signal from base station B as illustrated by theexample diagram shown in FIG. 6;

FIG. 22 is a schematic block diagram of a receiver according to anembodiment of the present technique;

FIG. 23 is a schematic block diagram of a Physical Layer Pipe (PLP)processor which appears in the receiver shown in FIG. 22;

FIG. 24a is a schematic block diagram illustrating a first example of anOFDM detector adapted in accordance with a further example embodiment ofthe present invention; FIG. 24b is a schematic block diagram of anequaliser of national broadcast modulation symbols of the OFDM detectorshown in FIG. 24a ; and FIG. 24c is a schematic block diagram of anequaliser of local service modulation symbols of the OFDM detector shownin FIG. 24 a;

FIG. 25 is a schematic block diagram of a second example of an OFDMdetector forming part of the receiver of FIG. 22 adapted in accordancewith a further example embodiment of the present invention;

FIG. 26 is a schematic block diagram of an equaliser of nationalbroadcast modulation symbols of the OFDM detector shown in FIG. 25;

FIG. 27 is a schematic block diagram of an equaliser/demapper forrecovering local service data symbols of the OFDM detector shown in FIG.25;

FIG. 28 is a schematic representation of a signal constellation diagramfor 16QAM showing an example mapping of data bits to modulation symbols;

FIG. 29 is a schematic block diagram of an equaliser/demapper forrecovering national broadcast data symbols of the OFDM detector shown inFIG. 25; and

FIG. 30 is a flow diagram illustrating an example operation of a processrequired to equalise a single frequency signal which includes componentsfrom a first and a second modulation scheme.

DESCRIPTION OF EXAMPLE EMBODIMENTS

As set out above embodiments of the present invention seek to provide,in one application, an arrangement in which local content can betransmitted within a single frequency network whilst allowing otherparts of the network still to receive a primary broadcast signal ortheir own local content. One example illustration is where local contentis required to be broadcast contemporaneously with a national broadcasttelevision programme.

FIG. 1 provides an example illustration of a network of base stations BSwhich are transmitting, via transmit antennas 1, a signal in accordancewith a commonly modulated OFDM signal. The base stations BS are disposedthroughout a geographical area within a boundary 2, which may be, in oneexample, a national boundary. As explained above in a single frequencynetwork configuration the base stations BS are all broadcasting the sameOFDM signal at the same time on the same frequency. Mobile devices M mayreceive the OFDM signal from any of the base stations. Moreparticularly, the mobile devices M may also receive the same signal fromother base stations because the signal is simultaneously broadcast fromall of the base stations within the area identified by boundary 2. Thisso-called transmit diversity arrangement is typical of a singlefrequency OFDM network. As part of the detection of the OFDM signals ina receiver which is recovering data from OFDM symbols, energy from thetransmitted OFDM symbols which is received for each symbol fromdifferent sources is combined in the detection process. Thustransmitting the same signal from different base stations can improvethe likelihood of correctly recovering the data communicated by the OFDMsymbols, provided that any component of the received OFDM symbol or echoof that OFDM symbol falls within a total guard interval period allowedfor the network deployment.

As shown in FIG. 1, in some examples the base stations BS may becontrolled by one or more base station controllers BSC, which maycontrol the operation of the base stations. In some examples the basestation controllers BSC may control one or more of the base stationswithin a part of the network associated with a geographical area. Inother examples the base station controllers BSC may control one or moreclusters of base stations so that the transmission of local content isarranged with respect to a time division multiplexed frames.

As mentioned above, the area identified by the boundary 2 couldcorrespond to a national boundary so that the network of base stationsis a national network. As such, in one example the television signalsbroadcast nationally are each transmitted from the base stations BSshown in FIG. 1. However embodiments of the present technique are aimedat addressing a technical problem associated with providing anarrangement for transmitting locally broadcast signals from some of thebase stations shown in FIG. 1 but not others. An example of such anarrangement might be if local broadcast news or traffic news which isassociated with a particular area is broadcast from some of the basestations but not others. In a multi-frequency network this is trivial,because the signals for the local broadcast maybe transmitted fromdifferent transmitters on different frequencies and therefore detectedindependently of what is broadcast from other base stations. However ina single frequency network a technique must be provided in order toallow for local service insertion of content for some of the basestations but not others or different local content at different basestations.

As mentioned above prior art document US 2008/0159186 discloses atechnique for combining two modulation schemes to form a modulationlayer for each of a plurality of data sources. A transmitter which isimplementing such an arrangement is shown in FIG. 2. In FIG. 2 data isfed from a first data pipe 4 and second data pipe 6 to a modulator 8,which modulates the data onto the sub-carriers to form an OFDM symbol.The modulation is performed in such a way that the data from the firstdata pipe 4 can be detected separately from the detection of the datafrom both the first and the second data pipes 4, 6. An OFDM symbolformer 10 then forms the OFDM symbol in the frequency domain as providedat the output of the modulator 8 and converts the frequency domain OFDMsymbol into the time domain by performing an inverse Fourier transformin accordance with a conventional operation of an OFDMmodulator/transmitter. The time domain OFDM symbols are then fed to aradio frequency modulator 12 which up converts the OFDM symbols onto aradio frequency carrier signal so that the OFDM signal may betransmitted from an antenna 14.

The technique disclosed in US 2008/0159186 is illustrated in FIGS. 3aand 3b . FIGS. 3a and 3b provide an illustration of signal constellationpoints in the complex plane comprising in-phase I and Quadrature-phase Qcomponents. The example signal constellation points shown in FIG. 3a isfor QPSK, whereas the example shown in FIG. 3b is for 16QAM. Inaccordance with the known technique for obtaining multi-layermodulation, data from two sources is modulated onto the signalconstellation points of a second modulation scheme. The signalconstellation points of the second modulation scheme represent thepossible modulation symbol values available for the modulation scheme.For the first modulation scheme shown in FIG. 3a , the signalconstellation points for QPSK are provided as small circles “o” 20. Assuch the bits from a source B that is provided from the source data pipe6 are mapped onto the signal constellation points as shown in FIG. 3a ,so that each possible modulation symbol value represents two bits fromthe source b0b1 in conventional manner using Grey coding for example.

The second modulation scheme shown in FIG. 3b is 16QAM, which provides16 possible signal constellation points 22 represented as “x”. Inaddition to the modulation of the signal by data from the first datapipe 6, which is shown as b0b1 a selection of one of the constellationpoints from each of the four quadrants shown in FIG. 3b also identifiesone of the four possible values for two bits from the second source datapipe 4 for the values a0a1. Thus detection of one of the signal pointsshown in FIG. 3b will not only identify a value for a0a1, but also avalue for b0b1 depending upon which of the four quadrants from which thesignal point is detected. Accordingly, a multi-layer modulation schemecan be made.

Transmitter

Embodiments of the present technique provide an arrangement whichutilises the multi-layer modulation technique according to US2008/0159186 to provide a local broadcast service for local contentwhilst still allowing base stations in neighbouring areas to detect anational broadcast signal.

A transmitter embodying the present technique, which might be used toinsert local content at one of the base stations shown in FIG. 1 isshown in FIG. 4. In FIG. 4 a plurality n of Physical Layer data Pipes(PLP) 30 are arranged to feed data for transmission to a scheduler 34. Asignalling data processing pipe 36 is also provided. Within each of thepipes the data is received for a particular channel from an input 38 ata forward error correction encoder 40 which is arranged to encode thedata, for example, in accordance with a Low Density Parity Check (LDPC)code. The encoded data symbols are then feed into an interleaver 42which interleaves the encoded data symbols in order to improve theperformance of the LDPC code used by the encoder 40.

The scheduler 34 then combines each of the modulation symbols from eachof the data pipes 30 as well as the signalling processing pipe 36 intodata frames for mapping onto OFDM symbols. The scheduled data ispresented to a data slice processing unit 50, 51, 52 which includes afrequency interleaver 54, a local pilot generator 180, a modulator 182,an optional MISO processing unit 184 and a pilot generator 56. The dataslice processor arranges the data for a given PLP in such a manner sothat it will occupy only certain sub-carriers of the OFDM symbol. Thedata output from the data slice processors 50, 51, 52 is then fed to aTime Division Multiple Access (TDMA) framing unit 58. The output of theTDMA framing unit 58 feeds an OFDM modulator 70 which generates the OFDMsymbols in the time domain which are then modulated onto a radiofrequency carrier signal by an RF modulator 72 and then fed to anantenna for transmission 74.

As explained above, embodiments of the present invention provide atechnique for allowing for local content to be broadcast from one ormore base stations within a local area relating to a national areacovered by the network shown in FIG. 1. To this end, the transmittershown in FIG. 4 also includes a local service insertion data sliceprocessor 80 which includes a frequency interleaver 54 and a local pilotgenerator 180. However, in addition, according to the present technique,the modulator 44 shown in the data slice processor 50 has a second inputfor receiving the data from the local service insertion data sliceprocessor 80. According to the present technique the modulator 44modulates the local service insertion data onto a related set of signalconstellation points according to a second modulation scheme. The signalconstellation points of the second modulation scheme, which is used forthe local content as well as the primary data, are related toconstellation points of the first modulation scheme which is used forjust communicating the primary data from the PLP pipe n as will beexplained with reference to FIGS. 5 and 6.

As shown in FIG. 4 the modulator 44 has a first input 82 which receivesdata from the data slice processor 50 and a second input 84 whichreceives data from the local service insertion data slice processor 80.In the following description the data from the data slice processor 50,will be referred to as the first or primary data pipe. In one examplethe data from the first data data slice processor 50 carries a nationalbroadcast channel, which would be communicated throughout the entirenetwork of FIG. 1.

The modulator 44 is shown in more detail in FIG. 5. As shown in FIG. 5the data from the local service insertion pipe 80 is fed from the secondinput 84 into a first data word former 90. The data from the first datapipe is fed from the first input 82 into a second data word former 92.The data from the first data pipe when received in the data word former92 is arranged to form four groups of bits y0y1y2y3 for mapping onto oneof 16 possible values of a 16QAM modulation symbol within a symbolselector 94. Similarly, the data word former 90 forms the data from thefirst data pipe 82 into data words comprising four bits y0y1y2y3.However, the data word former 90 also receives the data symbols from thelocal service insertion pipe 80 and so appends two of the bits from thelocal service insertion data pipe 84 to the data bits from the firstdata pipe 82 to form a six bit data word y0y1y2y3h0h1, which is fourbits y0y1y2y3 from the symbol stream from the first data pipe 32 and twobits h0h1 from the local service insertion pipe 80, thus forming a sixbit word for selecting one of 64 possible modulation symbol values of64QAM (2⁶=64).

A symbol selector 96 is arranged to receive the six bit wordy0y1y2y3h0h1 and in accordance with the value of that word select one ofthe 64 possible values of the 64QAM modulation scheme to form at anoutput 96.1 a stream of 64QAM symbols. The respective outputs from thesymbol selectors 94, 96 are then fed to a switch unit 98 which alsoreceives on a control input 100 an indication as to when the localcontent received from the local service insertion pipe 90 is present andis to be broadcast from the base station. If the local service insertiondata is to be broadcast from the base station then the switch 98 isarranged to select the output 96.1 from the 64QAM symbol selector 96. Ifnot then the switch is arranged to select the output 94.1 from the 16QAMsymbol selector 94. Modulation symbols are therefore output from themodulator 44 for transmission on the OFDM symbols on an output channel102.

The control input 100 may provide, in some examples, a control signalwhich indicates when local content is being transmitted from the localservice insertion data slice processor 80. The control signal providedin the control input 100, may be generated from a base stationcontroller to which the transmitter within the base station isconnected.

In other examples the signalling data processing pipe 36 may be arrangedto communicate via L1 signalling data an indication to when the localservice insertion pipe 80 is or will be transmitting the local data.Thus a receiver may recover may detect and recover the L1 signallingdata and determine when or whether the local content is being or will betransmitted. Alternatively, the receiver may be provided with a dataproviding a schedule of when the local content data is to betransmitted, by some other means, such as by pre-programming thereceiver.

Deployment of Base Stations

FIG. 6 provides an example illustration of an arrangement which may beproduced within FIG. 1 in which a first base station BS 110 may transmitdata from the first data pipe 32 within a cell A, whereas a neighbouringbase station BS 112 transmits data within a second cell B, thetransmitted data including data from the first data pipe 32 but also thelocal service insertion data from the local service insertion pipe 80.Thus the base station 110 from the cell A is transmitting an OFDM symbolwith sub-carriers modulated using 16QAM whereas the base station 112from the cell B is transmitting the OFDM symbols by modulatingsub-carriers with 64QAM. Thus as shown in FIG. 6 as the bit orderingshows, the final two bits h0h1 are used to select a finer detail of asignal constellation point according to 64QAM whereas the bits y0y1y2y3are used to select one of the 16QAM symbols in a coarser grid within thecomplex plane.

As already explained, both of the base stations 110, 112, within thecells A and B will be transmitting the OFDM symbols contemporaneously onthe same frequency. As such a receiver in a mobile terminal will receivea combined OFDM signal as if, in part, the signal was being received viadifferent paths in a multi-path environment. However, the OFDM signaltransmitted from base station 110 within cell A comprises OFDM symbolsmodulated using the first modulation scheme 16QAM whereas the OFDMsymbols transmitted from the base station 112 within cell B will bemodulated using the second modulation scheme 64QAM. At the receiverwithin the mobile terminal, a proportion of the total power with whichthe OFDM symbols are received with the first modulation scheme and thesecond modulation scheme will depend on the proximity of a mobile deviceM to each of the transmitters within the cells A and B. Furthermore, thelikelihood of correctly recovering the data symbols from the first datapipe and the local service insertion pipe will depend on the extent towhich the receiver can detect OFDM symbols according to the firstmodulation scheme 16QAM transmitted from cell A or OFDM symbolsaccording to 64QAM transmitted from cell B in the presence of OFDMsignals modulated with the second and the first modulation schemesrespectively.

As shown in FIG. 7 three plots 120, 122, 124 of possible simulatedsignal constellation values are shown for an example of 16QAM and 64QAMwhich are shown for example in FIG. 8. The first left hand plot 120provides a plot in the complex plane of received modulation symbolvalues when the transmitters in the base stations 110, 112 of cells Aand B are transmitting OFDM symbols with sub-carriers modulated with16QAM and 64QAM modulation schemes respectively, because cell B istransmitting local service insertion data. The first plot 120corresponds to a mobile device being at position X for which it isassumed that 80% of the received signal power is from cell A and 20% ofthe received signal power is from cell B. As can be seen in FIG. 7 theplot 120 provides discrete signal points in accordance with a 16QAMreceived signal, but with an apparent increase in noise as a result of aspread of possible points caused by the 20% power coming from the cell Bwhich is transmitting 64QAM modulation symbols.

Correspondingly, a middle plot 122 provides a plot of signal values inthe complex plane when the receiver is at position Y and for which it isassumed that 60% of the received power is from cell A and 40% of thereceived power is from cell B. As can be seen, although the signalconstellation plots are grouped into clusters corresponding to anassociation with each of the possible values of a 16QAM symbol, discreteconstellation points have been formed in accordance with a 64QAMmodulation scheme. Thus it will be appreciated that if the signal tonoise ratio is high enough then a receiver at position Y can detect oneof the 64QAM signal points and therefore recover the local inserteddata. Correspondingly, a right hand plot 124 illustrates the case atposition Z, for which it is assumed, for example, that only 10% of thesignal power comes from the cell A and 90% of the signal power comesfrom cell B. Therefore, as shown in the plot 124, clearly each of the64QAM signal constellation points are available for detecting andrecovering data, which is produced for both the first data pipe and thelocal service insertion data pipe. Accordingly, it will be appreciatedthat depending on the position of the receiver, a mobile terminal canrecover the locally transmitted data and the data transmitted from thefirst data pipe (for example the national broadcast) when in or aroundcell B, whereas in cell A a receiver will still be able to recover thedata from the first data pipe. Therefore an effect of using the layeredmodulation provided by the second modulation scheme of a 64QAM signaland the first modulation scheme 16QAM will not disrupt the reception ofthe nationally broadcast data when locally broadcast data is transmittedfrom a neighbouring cell.

TDMA Local Service Insertion

A further enhancement which some embodiments of the present techniquemay use is to distribute the capacity for local service transmissionbetween a cluster of neighbouring cells to the effect that the localcontent transmitted using the higher order (second) modulation scheme istransmitted at different times in different cells. This technique isillustrated with reference to FIGS. 9a, 9b and 9 c.

In FIG. 9a a cluster of four cells is shown. These are shown withdifferent grades of shading and are labelled respectively Tx1, Tx2, Tx3,Tx4. Thus FIG. 9a illustrates a cluster of four cells. As will beappreciated in addition to receiving the data from the first data pipe,which may be for example the national broadcast channel, a regionalbroadcast may also be provided using the local data insertion pipe incombination with the higher order hierarchal modulation technique asexplained above. However as explained above when the second or higherorder modulation technique is being used, the effect is to introducenoise or interference which reduces the signal to noise ratio forreceivers receiving the data from the first communications channel thatis the national broadcast using the first or lower order modulationscheme. More specifically, for example, if the national broadcast signalfrom the first data pipe is modulated using QPSK and the combined firstcommunications channel and the local service insertion channel aremodulated onto the second or higher order modulation scheme of 16QAMthen the 16QAM broadcast will appear as an increase in noise for areceiver trying to receive the OFDM symbols modulated with the QPSKmodulation scheme.

In order to reduce the amount of interference caused by thesecond/higher order modulation scheme (16QAM) with respect to thefirst/lower order modulation scheme (QPSK) the cells which broadcast theOFDM signals are clustered as shown in FIG. 9a . Furthermore thetransmitters within the four cell cluster illustrated in FIG. 9a taketurns on a frame by frame basis to broadcast the higher order 16QAMmodulation signal providing data symbols from the first datacommunications pipe and their local service insertion pipe. Such anarrangement is illustrated in FIG. 9 b.

In FIG. 9b a TDMA frame composed of four physical layer frames is shown.The physical layer frames are labelled frame 1, frame 2, frame 3 andframe 4. Within each physical layer frame the OFDM signals arecommunicating data from various PLPs. As explained abovecontemporaneously with the transmission of the data for the first datapipe using QPSK, OFDM symbols carrying data from both the first datapipe and the local service insertion pipe are also transmitted using forexample 16QAM. However in order to reduce the interference caused by the16QAM modulation only one of transmitters Tx1, Tx2, Tx3, Tx4 within thecluster of four cells is allowed to transmit OFDM symbols with thehigher order 16QAM modulated sub-carriers during each physical layerframe of the TDMA frames. Thus in physical layer frame 1, only Tx1transmits the OFDM symbols with sub-carriers modulated with 16QAM toprovide data from the combined first data pipe and its local serviceinsertion pipe, whilst in frame 2 only transmitter Tx2 transmits theOFDM symbols with 16QAM, and thereafter TX3 in frame 3 and TX4 in frame4. Then the pattern repeats for the next TDMA frame. In each case, allother transmitters are transmitting OFDM symbols modulated with QPSK orthe constellation used for carrying only the first data pipe.

As a result of time dividing the transmission of the local serviceinsertion data between each of the four transmitters Tx1, Tx2, Tx3, Tx4,effectively the local data rate is a quarter of that of the first datapipe. Thus each cell transmits local service insertion content everyfourth physical layer frame. However correspondingly because the higherorder modulation scheme is only transmitted from a cell once in everyfour frames, the effective interference experienced by receivers locatedin the coverage area of the four cells that wish to receive thefirst/lower order modulations scheme (QPSK) is correspondingly reduced.Thus in a pattern of cells illustrated in FIG. 9c , the interferencewhich is caused by the local service insertion data and would appear asincreased noise to the receiver is distributed throughout the cluster offour cells. Therefore the relative interference or increasing noisecaused by the local service insertion data is reduced. This can beconsidered to be the equivalent of frequency re-use in a multi frequencynetwork. For the example illustrated in FIG. 9a, 9b, 9c , the followingtable represents the transmission of OFDM symbols with each of the first(16QAM) and second (64QAM) modulation schemes:

Frame 1 Frame 2 Frame 3 Frame 4 Tx1 64 QAM 16 QAM 16 QAM 16 QAM Tx2 16QAM 64 QAM 16 QAM 16 QAM Tx3 16 QAM 16 QAM 64 QAM 16 QAM Tx4 16 QAM 16QAM 16 QAM 64 QAM

Table illustrating the modulation of OFDM symbols, when the localservice insertion data is modulated using a second/higher modulationscheme of 64QAM and the first/lower order modulation scheme is 16QAM forcarrying data symbols from the first/national data pipe.

As will be appreciated, a result of allocating the transmission of thelocal content over a cluster of four TDMA frames between a cluster offour base stations, may be to reduce the bandwidth for the local contentservice by one quarter, if a receiver is only able to receive the OFDMcarrying signal from one base station only, which will typically be thecase. The allocation of the local content to the transmitter of the basestation in each cluster may be provided for example via signalling dataprovided by the signalling data pipe.

Although in the example provided above the cells are clustered intogroups of four, it will be appreciated that any number can be used.Advantageously the cells are grouped into clusters of four to provide abalanced trade-off between an amount of baseband bandwidth (bit rate)afforded to the local service insertion service and an amount ofreduction in the signal to noise ratio caused to the reception of datafrom the first data pipe using the lower order modulation scheme by thetransmission of the higher order modulation scheme carrying data fromboth the first data pipe and the local service insertion channel. Assuch a cell structure shown in FIG. 9c can be used to transmit localcontent every fourth physical layer frame for a different group of fourcells and the arrangement of the cell clustering repeated throughout torepresent an equivalent arrangement of frequency re-use.

According to the present technique the transmitter within the basestations shown in FIG. 4 may be adapted to implement the TDMA framestructure illustrated above. In one example, the scheduler 34 forforming the modulated sub-carrier signals into the OFDM symbols and aframing unit 58 may be arranged to schedule the transmission of the OFDMsymbols according to the time divided frame illustrated in FIG. 9b . Thescheduler 34 and the framing unit 58 are arranged to transmit OFDMsymbols which are carrying data symbols from both the first data pipeand the local service insertion pipe using the second modulation schemeas illustrated in the table above.

Equalisation of Combined Local Service Insertion and National BroadcastSignals

A further aspect of the present technique will now be described withreference to FIGS. 10 to 15. As explained above, data from a localservice insertion channel is transmitted with data from a nationalbroadcast channel using a higher order modulation scheme such as 16QAM,whereas data from the national broadcast channel is transmitted using alower order modulation scheme such as QPSK. A mobile receiver which isable to detect the local service insertion data which is conveyed withthe data from the national broadcast channel by a 16QAM modulationscheme may be required to detect the 16QAM signal in the presence of aQPSK signal, which conveys data from the national broadcast channelonly. The 16QAM modulation scheme conveying data from the nationalbroadcast channel and the local broadcast channel and the QPSKmodulation scheme conveying the national broadcast channel arerepresented in FIGS. 3a and 3b and described above. In the followingdescription the higher order modulation scheme which is conveying dataaccording to the national broadcast channel and the local serviceinsertion channel will be referred to as the local service insertionchannel or data and the national broadcast channel will be referred asthe national broadcast channel, data or signal.

A further ancillary problem addressed by an embodiment of the presenttechnique is to provide a receiver which can equalise a signal receivedat the receiver which is a combination of the local service insertionsignal that is the 16QAM signal and the national broadcast signal thatis the QPSK signal for example. Equalising a signal which is acombination of a national broadcast signal and a local service insertionsignal, which is a combination of a 16QAM and a QPSK signal is thereforeaddressed by a further aspect of the present technique.

As shown in FIG. 10 a mobile receiver M is located at a positionapproximately equi-distant from the base station transmitting the localservice insertion signal 112 and a base station transmitting thenational broadcast signal 110. Thus the signal received by the mobilereceiver M is comprised of a combination of the local service insertionsignal s(t)+d(t) convolved with the channel NO between the local serviceinsertion base station 112 and the mobile receiver M and the nationalbroadcast signal s(t) convolved with a channel h_(n)(t) from thenational broadcast base station 110 and the mobile receiver M. Thus thereceived signal r(t) is represented by the following equation (where thesymbol ‘*’ represents convolution):

$\begin{matrix}{{r(t)} = {{{h_{n}(t)}*{s(t)}} + {{h_{l}(t)}*\left\lbrack {{s(t)} + {d(t)}} \right\rbrack}}} \\{= {{{s(t)}*\left\lbrack {{h_{n}(t)} + {h_{l}(t)}} \right\rbrack} + {{d(t)}*{h_{l}(t)}}}}\end{matrix}$

Following an FFT in which the received signal is transformed into thefrequency domain, the signal formed at the output of the FFT is:R(z)=S(z)[H _(n)(z)+H _(l)(z)]+D(z)H _(l)(z)

A signal constellation therefore can be represented in the complex planefor the national broadcast signal as shown in FIG. 11a , and the localinsertion signal as shown in FIG. 11b ; the national broadcast signalbeing QPSK as shown in FIG. 11a and the local service insertion signalbeing 16QAM shown in FIG. 11b . Thus the national broadcast signal ofFIG. 11a provides a lower order modulation scheme with respect to thehigher order modulation scheme of 16 QAM shown in FIG. 11b . However,the representation of the signals shown by the constellation points ofFIGS. 11a and 11b are without noise and moreover, without the presenceof either of the other signals.

FIGS. 12a and 12b provide a corresponding representation of the signalconstellation in the complex plane where the mobile receiver M receivesa signal in the presence of both the national broadcast signal s(t) andthe locally broadcast signal s(t)|d(t) and where the channel responsesH_(n)(z) and H_(l)(z) are not equal. In FIG. 12a the signalconstellation of R(z) the combined signal as expressed above is acombination of the national broadcast signal and the local broadcastsignal. FIG. 12b shows the effect of dividing the received signal R(z)by [H_(n)(z)+H_(l)(z)] which is a combination of the channels from thebase station of the national broadcast signal 110 and the channel of thelocal insertion base station 112, to produce C(z). The diagram in FIG.12b is assuming perfect channel estimation and without noise. As can beseen from FIG. 12b only a small amount of noise will be required inorder to cause a false detection of a particular modulation symbol ofthe local broadcast signal. The division of R(z) by the combined channelforms an equalised signal C(z):

$\begin{matrix}{{C(z)} = \frac{R(z)}{\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}} \\{= {{S(z)} + {\frac{H_{l}(z)}{\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}{D(z)}}}}\end{matrix}$

However we do not know H_(n)(z) and H_(l)(z) separately, and so thefollowing cannot be computed:

$\frac{H_{l}(z)}{\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}$

According to the present technique in order to recover the localinsertion signal from the national broadcast signal, it is necessary todetermine either the channel H_(n)(z) from the national base station 110or the channel H_(l)(z) from the local service insertion base station112 separately. With knowledge of either the national broadcast channelH_(n)(z) or the local insertion channel H_(l)(z) it would be possible tocompute the term D(z). Thus, first detecting the national broadcastsignal using the lower order modulation scheme and subtracting thedetected signal from the received signal it is then possible withknowledge of either the channels from the national broadcast basestation H_(n)(z) or the local service insertion signal base stationH_(l)(z) to recover the local signal D(z). Thus, according to thepresent technique the term H_(l)(z)D(z)/[H_(n)(z)+H_(l)(z)] is treatedas noise and the national broadcast data is recovered by slicing S(z) togive an estimate of the national broadcast signal Ŝ(z). Accordingly, bycalculating the composite channel [H_(n)(z)+H_(l)(z)] from the nationalbroadcast base station H_(n)(z) and the local service insertion signalbase station H_(l)(z) and convolving with the estimate of the nationalbroadcast signal (by multiplication in the frequency domain) it ispossible to subtract this combination from the received signal to forman estimate of the local service insertion signal convolved with thechannel from the local service insertion base station.

Therefore to detect the local service insertion signal, the followingsteps are required:

-   -   1. Estimate S(z) as Ŝ(z) by considering

$\frac{H_{l}(z)}{\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}{D(z)}$as noise when slicing S(z);

-   -   2. The equaliser has already computed [H_(n)(z)+H_(l)(z)] as the        combined channel from the national OFDM reference pilots;    -   3. Compute D(z)H_(l)(z)≈R(z)−Ŝ(z)[H_(n)(z)+H_(l)(z)]; which        provides a complex signal as shown in the complex plane diagram        in FIG. 13 a;    -   4. If some of the D(z) are known from additional pilots provided        in the local service insertion signal, then H_(l)(z) can be        estimated to give Ĥ_(l)(z)    -   5.

${{\hat{H}}_{l}(z)} \approx \frac{{R(z)} - {{\hat{S}(z)}\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}}{D(z)}$

-   -   6. Interpolation can be performed on Ĥ_(l)(z) in the frequency        direction to form H_(l)(z) and so    -   7.

${\overset{\sim}{D}(z)} \approx \frac{{R(z)} - {{\hat{S}(z)}\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}}{{\hat{H}}_{l}(z)}$

Thus, by cancelling the channel from the local service insertion basestation Ĥ_(l)(z), a signal constellation diagram shown in FIG. 13b isformed from which the local service insertion data {tilde over (D)}(z)can be recovered.

As will be appreciated from the above explanation in order to recoverthe local service insertion signal {tilde over (D)}(z) it is necessaryto estimate the local service insertion channel Ĥ_(l)(z) from the localservice insertion base station which is separate from the channel fromthe national broadcast base station H_(n)(z).

In a further embodiment, the computed {tilde over (D)}(z) can be used toget a better estimate of Ŝ(z) by computing the following:R(z)−{tilde over (D)}(z)H _(l)(z)=S(z)[H _(n)(z)+H _(l)(z)]Then divide each side by [H_(n)(z)+H_(l)(z)] and slice again for Ŝ(z).This kind of iteration may be continued many times to get a continuousimprovement in the estimate of {tilde over (D)}(z).

The above explanation has been provided to give an explanation of thegeneral technique by which the data from the national broadcast signaland the local broadcast signal can be recovered. As given above a simpletechnique for cancelling the effects of a channel from a received signalis to divide the received signal by the estimate of the channel. Howeverthere are other equalisation techniques which can be used are some arepresented below after the results section which includes generating alog likelihood ratio for the received data symbols. This exampletechnique avoids a potential problem caused by cancelling by division ifthere are nulls in the frequency domain, which produces noiseamplification as a result of dividing by zero.

Estimating Local Channel Using Local Pilots

According to the present technique the channel from the local serviceinsertion base station H_(l)(z) is estimated by including local serviceinsertion pilot symbols on selected sub-carriers which are transmittingthe local service insertion modulation symbols. Such an arrangement isshown in FIGS. 14a, 14b and 14 c.

In FIG. 14a an illustrative representation of an OFDM symbol in thefrequency domain is provided showing a plurality of subcarriers whichare then designated for conveying data according to the nationalbroadcast signal s(t) and subcarriers which are dedicated totransmitting pilot symbols Ps in accordance with a conventionalarrangement. FIG. 14b provides an illustration of an OFDM symbol inwhich local service insertion symbols are introduced on top of thenational broadcasting symbols using the hierarchical modulation scheme.However, in order to estimate the channel via which the local serviceinsertion symbol is broadcast, it is necessary to select some of thesubcarriers which are carrying data according to the local serviceinsertion and replace these symbols with known symbols which will act aspilot symbols Pd. Such an arrangement is shown in FIG. 14c .Accordingly, it will be appreciated that the local service insertionpilots Pd can be transmitted in place of symbols which would betransmitted on subcarriers with higher order modulation symbols whichwould be arranged to carry the local service insertion data butarranging for these to be replaced by known symbols. Therefore thesesub-carriers can convey a known symbol for the higher order modulationwhich can act as a pilot Pd. However, as will be appreciated in order totransmit the local service insertion signal pilots Pd. it is necessaryto accommodate the frequency interleaving which would be required for aconventional transmission of the local service insertion data.

As shown in FIG. 4, according to the present technique at the output ofthe frequency interleaver 54 for each data slice processor 50, 51, thedata slice processors 50, 51 which include local service insertion datainclude a block 182 for inserting the local service insertion pilots Pdbefore generating the hierarchical modulation symbols as formed by themodulators shown in FIG. 4. The modulators 182 are arranged to map thedata symbols onto modulation symbols in accordance with the hierarchicalmodulation scheme being used. Optionally, where a multiple input singleoutput (MISO) scheme is being employed then further processing of thepilots is performed as illustrated by the MISO block 184. Following theMISO block 184, the pilot symbols are inserted on separate pilotsubcarriers via the main pilot insertion unit 56 following which theframing unit 58 forms the OFDM symbols in the frequency domain in acombination with the OFDM block 70.

As shown in FIG. 4 at the output of the frequency interleaver 54 in abranch of the signal insertion data slice processor, the local serviceinsertion data which is produced after the frequency interleaver 54 isfed to the local pilots insertion block 180 in which the data symbolsfor the local service insertion are replaced by the pilot symbols eitherby puncturing or for example where the modulation symbols which are tobe used to carry the local service insertion of pilots are left vacantbetween data cells or are moved to accommodate the local serviceinsertion pilots. As will be appreciated the local service insertionpilots Pd are pre-designated and so can either be reserved for localservice insertion pilots or the data can be moved to accommodate thelocal service insertion pilots. Thus, the arrangement substantially asrepresented in FIG. 14c is produced at the output of the QAM modulator182.

FIG. 15 provides a schematic block diagram which corresponds to theschematic block diagram shown in FIG. 4 except that FIG. 15 provides anexample in which a multiple-input multiple-output (MIMO) transmissionscheme is being used. However, a complication with the arrangement for aMIMO scheme is that the local service insertion pilots Pd, which areformed as part of the hierarchical modulation structure must be insertedbefore the frequency interleaver 192. This is because for a MIMO scheme,the pilots on each version of the OFDM signal to be transmitted areadapted with respect to each other and so each of the versions must beformed separately for each version. This applies for both the nationalbroadcast modulation symbols and also the local service insertionsymbols. Accordingly, it is not possible to combine the local serviceinsertion pilots at the output of the frequency interleaver 54.

According to the present technique, in order to accommodate anarrangement in which the local service insertion pilots are formed inthe signal before the frequency interleaver 54 then the local serviceinsertion pilots are arranged with respect to the subcarriers which areconveying the hierarchical modulated data in a block 190 which is thenfed to a frequency de-interleaver 192 which performs an inverse of theinterleaving performed by the frequency interleaver 54. Thus, the pilotsub-carriers which include the local service insertion pilots Pd arearranged at their desired position and the frequency de-interleaver,de-interleaves these modulation symbols before the local serviceinsertion data is applied by a local service insertion data block 194.At the output of the QAM modulator 182, the modulation symbols areformed and fed to a MIMO block 184. The frequency interleaver 54 thenperforms a mapping which is a reverse of the de-interleaver mappingperformed by the frequency de-interleaver 192 so that at the output ofthe frequency interleaver 54, the local service insertion pilots areonce again at the desired locations on the designated sub-carriers forthe local service insertion pilots. Accordingly, OFDM symbols are formedwith the local service insertion pilots Pd at their desired location.The main pilots Ps for the national broadcast signal are then added atthe sub-carrier positions concerned via the main pilot insertion block56 before the framing unit 58 and the OFDM unit 70 form the OFDM symbolsas per a conventional arrangement.

Thus, according to the present technique the local service insertionpilots Pd are arranged at the desired location by first arranging forthem to be disposed at their desired location and then forming aninverse of the interleaving using a de-interleaver so that wheninterleaved they are once again arranged at their desired location.

A received architecture which is arranged to recover the local serviceinsertion data or the national broadcast data is described below withreference to FIG. 24.

Results

Various results are provided in FIGS. 16 to 21 for exampletransmitter-receiver chains operating with different forward errorcorrection encoding rates of rate ½, ⅗, ⅔ and ¾, and for a firstmodulation scheme of 16QAM, a second modulation scheme of 64QAM. FIGS.16, 17, 18, 19, 20 and 21 provide examples for different ratios of thepower from cell A and cell B. For FIG. 16 the fraction of the power ofthe received signal from cell A is 99% and 1% from cell B. The relativedelay between time of arrival from cells A and B is 4.375 us. For FIG.16 80% of the power is from cell A and 20% is from cell B with a 2.2 μsdelay in time of arrival from cell B. FIG. 17 provides a 99% power fromcell A and 1% of power from cell B at a 0 μs delay in relative time ofarrival. FIG. 18 shows 60% of power from cell A and 40% of power fromcell B at a 0 μs relative delay and FIG. 19 shows a 50% power from basestation A and 50% power from cell B at a 0 μs relative delay. Finally,FIG. 20 shows results in a situation where 10% of the power is from cellA and 90% is from cell B with the signal from cell A arriving thereceiver 2.2 μs after the arrival of the signal from cell B. As can beseen from the example in FIG. 21 there is insufficient signal to noiseratio to decode the ⅗, ⅔ rate codes. The required SNR should be thatenough for the decoding of 64QAM. With respect to each of the plots isshown a signal to noise ratio value which would correspond to asituation in which the transmitter for the same neighbouring cell wasnot transmitting the local service insertion data on the higher ordermodulation scheme 64QAM for this example. Where appropriate some of theplots include points for each of the respective coding rates of ½, ⅗, ⅔and ¾ at a bit error rate of 10⁻⁷ as represented as a “⋄”. As shown ineach case there is an increase in the signal to noise ratio required inorder to reach the same bit error rate value. However the performance ofthe scheme would still seem to be acceptable.

Receiver

A receiver which may form part of a mobile device for receiving thesignals broadcast by any of the base stations of the network shown inFIG. 1 will now be described. An example architecture for a receiver forreceiving any of the transmitted PLP pipes shown in FIG. 4 is providedin FIG. 22. In FIG. 22 a receiver antenna 174 detects the broadcastradio frequency signal carrying the OFDM signals which are fed to aradio frequency tuner 175 for demodulation and analogue to digitalconversion of a time domain base band signal. A frame recovery processor158 recovers time division multiplex physical layer frame boundaries andOFDM symbol boundaries and feeds each of the symbols for each of eachphysical layer frame to an OFDM detector 150. The OFDM detector 150 thenrecovers the national broadcast data and local service insertion datafrom the OFDM symbols in the frequency domain. The recovered nationalbroadcast data and local service insertion data is then fed to ade-scheduler 134 which divides each of these symbols into therespectively multiplexed PLP processing pipes. Thus the de-schedulerreverses the multiplexing of applied by the scheduler 134 shown in FIG.4 to form a plurality of data streams, which are fed respectively to PLPprocessing pipes 129, 130, 136. A typical receiver would have only asingle PLP processing pipe as each PLP may carry a full broadcastservice and this PLP processing pipe processes the data from any nationbroadcast PLP or any local service insertion PLP. The processingelements forming part of the PLP processing pipes shown in FIG. 22 isshown in FIG. 23.

In FIG. 23 the first example PLP processing pipe 130 is shown to includea QAM demodulator 144, a de-interleaver 142 and a forward errorcorrection decoder 140 which are arranged to substantially reverse theoperations of the QAM modulator 44, the interleaver 42 and the FECencoder 40 of FIG. 4. Optionally, the PLP processing pipe 130 may alsoinclude a MISO/MIMO detector 46 for performing multiple input multipleoutput or multiple input signal output processing. In operationtherefore modulation symbols are received at an input 200 and fed to theMISO/MIMO processor 146 whose role is to decode the space-time code thatwas used at the transmitter thereby producing one stream of modulationsymbols into a signal symbol stream which are then fed to the QAMdemodulator 144. The QAM demodulator detects one of the constellationpoints in the QAM modulation scheme used and for each detected pointrecovers a data word corresponding to that point. Thus the output of theQAM demodulator 144 is a data symbol stream which is fed to thede-interleaver 142 for de-interleaving the data stream from a pluralityof OFDM symbols or from within an OFDM symbol.

Since the data symbols have been encoded in the transmitter shown inFIG. 4, for example, using a low density parity check code, the symbolsare decoded by the FEC decoder 140 to form at an output 202 base banddata stream for the PLP.

In accordance with the present technique in some embodiments, thede-scheduler 150 is arranged to apply the TDMA frame in accordance witha cluster of base stations described above to recover OFDM symbols whichhave been modulated with the second modulation scheme and transmitted onone of the physical layer frames. Thus in accordance with the signaltransmission arranged for the cell cluster the receiver tries therecovery of the OFDM symbols with sub-carriers modulated in accordancewith the second modulation scheme in accordance with the frame timingapplied by the transmitter in the base station. The information as towhich physical layer frames carry hierarchical modulation for the givenPLP is carried in the signalling PLP which the receiver first receivesand decodes before any payload carrying PLP.

Equalising Received Single Frequency Signal

First Example OFDM Detector

As explained above, according to the present technique the OFDM detector150 shown in FIG. 22 is arranged to detect both the national signal S(z)and the local service signals D(z) using the pilot signals transmittedwith both the national and local signals and the additional pilotstransmitted with the local signal. Two example implementations of theOFDM detector 150 will now be described, with reference to FIGS. 24 to29.

FIG. 24a provides a representation of a schematic block diagram of afirst example of the OFDM detector 150 shown in FIG. 22. This can beused for a SISO, MISO or MIMO scheme. In FIG. 24 a Fast FourierTransform FFT block 290 converts the received signal from a time domaininto the frequency domain. A national broadcast signal equaliser 292then receives the frequency domain OFDM symbols and forms an estimate ofthe combined local service insertion channel and the national broadcastchannel as well as the received national broadcast data. Blocks whichmake up the single frequency network equaliser 292 are shown in FIG. 24b.

As shown in the FIG. 24b the single frequency network equalisercomprises a pilot separator 296 which separates the pilots from thereceived frequency domain signal. The frequency domain signal is fed atan output 298 of the pilot separator 296 to a divider circuit 300. Froma second output 302 of the separator 296 the pilot sub-carriers aredemodulated, interpolated in time by a time interpolation unit 304 andinterpolated in frequency by a frequency interpolation unit 308 to format an input 310 to the divider 300 an estimate of the combined nationalbroadcast channel and the local service insertion channel(H(z)_(l)+H_(c)(z)) so that the output of the divider forms a signalrepresentative of the national broadcast signal S(z) 312.

As shown in the receiver chain a de-mapper 314 then interprets thereceived modulation signals by slicing the modulation signalling aboutthe real and imaginary plane to detect an estimate of the nationalbroadcast signal Ŝ(z). The signal representative of the nationalbroadcast signal S(z) 312 is then fed to a frequency de-interleaver 316and then to a de-scheduler 134 as explained above for a general datarecovery of the national broadcast signal.

FIG. 24c provides an example implementation of a local equaliser 320. Asshown in FIG. 24c , the detected combined local service insertionchannel and national broadcast channel (H(z)_(l)+H_(c)(z)) are fed on anoutput 311 to a first input of a local equaliser 320. The estimate ofthe national broadcast symbols Ŝ(z) 315 is fed to a multiplier 322 whichreceives on a second input the estimate of the combined local serviceinsertion channel and the national broadcast channel 310. A subtractionunit 324 then subtracts the multiplication of the estimate of thenational broadcast symbols multiplied with the combined local serviceinsertion and national broadcast channels from the received signal toform an estimate of the local service insertion symbols which are fed toa local equaliser 320. The internal structure of the local equaliser 320is similar to that of the national broadcast signal equaliser. At theoutput of the local service insertion pilot separator 326 the pilotsignals are fed on a output 328 to a pilot demodulator 330 and then to atime interpolation unit 332 followed by a frequency interpolation unit334 which forms an estimate of the channel through which the localservice insertion symbols have passed. The estimate of the local serviceinsertion data is fed on an input 336 to divider circuit 338 whichreceives on a further input from the pilot separator 326, 340 the localservice insertion symbols and forms at an output 342 an estimate of thelocal service insertion data symbols. A de-mapper 344 and frequencyde-interleaver 346, then form an estimate of the data representing thelocally inserted data which is fed to the de-scheduler 134. Thereafter,the data recovery of the locally inserted data corresponds to that shownwith respect to the data pipe shown in FIG. 23.

As will be appreciated a further aspect of the present techniqueprovides a first estimate of the national broadcast data, which is thenrefined, based on the determination of the local service insertionsymbols to form a further refined estimate of the national broadcastsymbols which may be further used to further calculate a refinedestimate of the local service insertion symbols. Thus, an iterativefeedback arrangement in the form of a turbo-demodulation can be formedto provide further improvements on the estimate of the received signals.

Second Example of the OFDM Detector

As will be appreciated by those familiar with equalisation techniques,the implementation of the single frequency network equaliser 292 and thelocal insertion equaliser 320 include a divider circuits 300, 338. Eachof the divider circuits in the respective equalisers 292, 320 cancel theeffect of the respective channels by division. For example in the singlefrequency equaliser 300, the channel H(z) is cancelled from thecomponent of the signal which includes the data for the national signalS(z) by dividing in the frequency domain the national broadcast signS(z) multiplied with the channel (convolved with the channel in the timedomain) by the estimate of the channel H′(z) in order to recover thenational broadcast signal. However a multi-path fading radio channel canproduce nulls in the frequency response as a result of the multiplepaths interfering destructively to cancel each other, thereby producinga zero component. As a result, a simple division of the received signalby the channel estimate can amplify noise as a result of a division byzero. A divide by zero (or a number close to zero) can therefore causethe estimate of a modulation symbol at a position corresponding to thechannel sample which is zero or similar to produce a maximum of thecomplex real and imaginary terms. As a result information which themodulation symbol carries, the modulating data will be lost. The secondexample of the OFDM detector 150 is therefore arranged to obviate thistechnical problem by considering the effect of the channel in thegeneration of the log likelihood function at the de-mapper instead ofthe divide by zero in order to recover the data from the receivedsignal. The equaliser therefore performs a detection of the data using aminimum mean squared error equalisation technique. The second example ofthe OFDM detector 150 is shown in FIGS. 25, 26, 27 and 28.

FIG. 25 provide the second example of the OFDM detector 150 whichcorresponds to the example shown in FIG. 24a and so only the differenceswill be described here for brevity. An FFT processor 290, frequencyde-interleavers 316, 346, a national signal equaliser 292.2, themultiplier 322 and the subtraction circuit 324 operate to performcorresponding functions to those of the correspondingly number unitsshown in FIG. 24a described above. The differences from the firstexample therefore reside in a national QAM de-mapper 400, the bitslicing de-mapper 314.2 and a local equaliser/QAM de-mapper 402.

As with the first example, the FFT processor 290 recovers a frequencydomain version of the received OFDM symbol, and feeds the OFDM symbol toa first national equaliser 292.2. The national equaliser 292.2 is shownin FIG. 26 and operates to generate an estimate of the combined channelH(z) through which the national broadcast signal and the local broadcastsignal were received. The national equaliser 292.2 operates as thesingle frequency equaliser 292 shown in FIG. 24b to generate an estimateof the national broadcast signal Ŝ(z) by dividing the received signal bythe estimated channel. The national equaliser 292.2 therefore operatesin correspondence with the national equaliser shown in FIG. 24b , butotherwise operates in the same way.

The first estimate of the national signal Ŝ(z) is fed to a bit slicingprocessor, which recovers a first estimate of the data convey by thenational signal Ŝ(z) using the bit slicing processor 314.2, whichoperates in the same way as the QAM demapper 314 shown in FIG. 24c .Correspondingly, the multiplier 322 and the subtraction circuit 324operates as shown in FIG. 24c to form at an input to the localequaliser/QAM demapper 404, a signal which corresponds to the localsignal convolved with the local channel (D(Z)Hl(z)). Therefore accordingto the operation of the FFT processor 290, the first national equaliser292, the bit slicing demapper 314.2, the multiplier 322 and thesubtraction circuit 324, the following are produced at the respectiveoutputs of these circuits:

The receive signal at the output of the FFT processor 290 is:R(z)=S(z)[H _(l)(z)+H _(N)(z)]+D(z)H _(l)(z), in which H(z)=[H _(l)(z)+H_(N)(z)]The output from the first national equaliser 292.2 on an output channel310 is then:H(z)=[H _(l)(z)+H _(N)(z)]Whereas on the output 312 from the first national equaliser, the resultof dividing the received signal by the channel H(z) produces:

${C(z)} = {{S(z)} + \frac{{D(z)}{H_{l}(z)}}{H(z)}}$

Therefore the output 315 of the bit slicing demapper, which process C(z)by slicing about the real and imaginary planes produces S′(z).Accordingly, R′(z) is reconstituted at the output of the multipliercircuit 322 according to:R′(z)=S′(z)H(z)+D(z)H _(l)(z)

So that the output of the subtraction circuit 324 becomes:R′(z)−S′(z)H(z)=D(z)H _(l)(z)

As shown in FIG. 25, the local equaliser/QAM demapper 402 operates torecover the local signal D(z) and the H′_(l)(z) using the known localsignal pilots within D(z). The local equaliser 402 is shown in moredetail in FIG. 27. As shown in FIG. 27, the local equaliser 402 isarranged to generate the estimate of the local channel H′_(l)(z), usingthe pilot separator 326, the demodulator 330, the time interpolationcircuit 332 and the frequency interpolation circuit 334 as explainedabove with reference to FIG. 24c . However, in FIG. 27, the localequaliser 402 includes a local 2D QAM demapper, which operates togenerate an estimate of the local signal D′(z), as explained in thefollowing paragraphs, by using a log likelihood function rather than adivision circuit. This is achieved by using a 1D or a 2D demapper ofD(z)H_(l)(z) using the estimated H′_(l)(z) as the channel transferfunction. This gives estimate D′(z).

Minimum Mean Square Error Local Equaliser

The local equaliser 320 shown in FIG. 24c generates the estimate of thelocal signal D′(z), by cancelling the effects of the local signalchannel using a divider circuit and then de-maps the modulation symbolsof the local signal which are produced at the output of the dividercircuit 338. In contrast, the local equaliser/demapper 402 uses a 2D or1D QAM demapper, which combines the estimate of the channel H′_(l)(z)and a sample of the received local signal D(z)H_(l)(z) to produce a loglikelihood ratio for each of the estimated modulation symbols. The loglikelihood ratios (LLR) of the modulation symbols can then be used in asubsequent error correction decoder which utilises soft decisioninformation to recover the data communicated from the local signal. Assuch, because the local equaliser/demapper does not perform a divisionof the received local signal by the estimate of the local channel, thedivide by zero problem explained above is avoided or at least reduced.The operation of the local signal demapper 404 which generates a loglikelihood ratio for each estimate modulation symbol or cell of the OFDMsignal is explained as follows:

Log Likelihood Demapper

Let a received modulation symbol estimate or cell be designated asr=I_(d)+jQ_(d). The demapper is arranged in operation to calculate asoft bit in the form of a log-likelihood ratio (LLR) for each of thebits carried by the cell. The LLR for the bit at position i of theconstellation label (b_(i)) can be computed as:

${L\left( b_{i} \right)} = {\ln\left\lbrack \frac{P\left( {b_{i} = \left. 1 \middle| r \right.} \right)}{P\left( {b_{i} = \left. 0 \middle| r \right.} \right)} \right\rbrack}$where:${{P\left( {b_{i} = \left. k \middle| r \right.} \right)} = {{\sum\limits_{x \in {C{({k,i})}}}{{P\left( x \middle| r \right)}\mspace{14mu}{for}\mspace{14mu} k}} = 0}},1.$

where C(k,:) is the set of constellation points for which the value(b_(i)) of the bit at position i is k. Thus with 16QAM for example:C(0,3)={0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111} i.e. the set ofconstellation labels for which the MSB (b₃) is zero.

In its most general form:

${P\left( x \middle| r \right)} = {\frac{1}{\sigma_{I}\sqrt{2\pi}}{\exp\left( {- \frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}}} \right)}*\frac{1}{\sigma_{Q}\sqrt{2\pi}}{\exp\left( {- \frac{{{Q_{d} - {\rho_{Q}Q_{x}}}}^{2}}{2\sigma_{Q}^{2}}} \right)}}$

Where σ is the noise standard deviation of I_(d) and Q_(d) accordinglyand the ρ are the respective channel fading coefficients for the I and Qchannels. Note that in general, the σ and ρ can be different if theI_(d) and Q_(d) come from different OFDM cells because of the use ofrotated constellations, for example. When rotated constellations are notused, the pairs of σ and ρ are of course the same and simplify thingsconsiderably.

The multiplier terms before the exponentials only scale and so the LLRis proportional to:

${L\left( b_{i} \right)} = {\ln\left\lbrack \frac{\sum\limits_{x \in {C{({1,i})}}}{\exp\left( {{- \frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}}} + \frac{{{Q_{d} - {\rho_{Q}Q_{x}}}}^{2}}{2\sigma_{Q}^{2}}} \right)}}{\sum\limits_{x \in {C{({0,i})}}}{\exp\left( {{- \frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}}} + \frac{{{Q_{d} - {\rho_{Q}Q_{x}}}}^{2}}{2\sigma_{Q}^{2}}} \right)}} \right\rbrack}$This can be simplified using the max-log approximation which postulatesthat:

${\ln\left\lbrack {\sum\limits_{k = 1}^{L}{\exp\left( a_{k} \right)}} \right\rbrack} = {\max\left( a_{k} \right)}$It also follows that:

${\ln\left\lbrack {\sum\limits_{k = 1}^{L}{\exp\left( {- a_{k}} \right)}} \right\rbrack} = {\min\left( a_{k} \right)}$Therefore,

${L\left( b_{i} \right)} = {{\underset{x \in {C{({0,i})}}}{in}\left\lbrack {\frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}} + \frac{{{Q_{d} - {\rho_{Q}Q_{x}}}}^{2}}{2\sigma_{Q}^{2}}} \right\rbrack} - {\underset{x \in {C{({1,i})}}}{in}\left\lbrack {\frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}} + \frac{{{Q_{d} - {\rho_{Q}Q_{x}}}}^{2}}{2\sigma_{Q}^{2}}} \right\rbrack}}$Which represents a so-called 2D de-mapper, in which

$\sigma^{2} = {\frac{1}{2^{m}\varphi}{\sum\limits_{x}\left\lbrack {I_{x}^{2} + Q_{x}^{2}} \right\rbrack}}$

where φ is the linear SNR/cell computed for the relevant cell in theequalised/demapper signal and m=½ log₂(M) being the number of bitsconveyed per constellation axis. Note that for rotated constellationsthe φ are different for I & Q and so the σ will also be different. Thesummation in the above equation for

$\sigma^{2} = {\frac{1}{2^{m}\varphi}{\sum\limits_{x}\;\left\lbrack {I_{x}^{2} + Q_{x}^{2}} \right\rbrack}}$is the well known average energy per symbol of the particular QAMconstellation.

The operation of the local equaliser/de-mapper therefore performs thefollowing computations for each input cell r=I_(d)+jQ_(d):

-   -   1. For each point x=I_(x)+jQ_(x) in the constellation, calculate        the distances |I_(d)−I_(x)|² and |Q_(d)−Q_(x)|² scaled by the        relevant noise terms. If this is done in a brute force fashion,        M of these would be needed for M-QAM. However, if these are        calculated per contour of the relevant QAM then only M/2        subtractions and square operations are needed followed by M        additions.    -   2. For bit position i in the constellation label, find out for        each constellation point x, if b_(i) is one or zero and consider        if the I&Q distance sum of x from r (computed in step 1) is the        minimum for the relevant set. In practice, for each        constellation type and for each bit position i the indices to        the points which are members of the sets C(0,i) and C(1,i) can        be pre-identified and tabulated.    -   3. Once the two minima are found, perform the subtraction in the        equation to get L(b_(i)).

On the other hand, the 2D-demapper LLR function can be expanded into:

${L\left( b_{i} \right)} = {{\underset{x \in {C{({0,i})}}}{in}\left\lbrack {\frac{I_{d}^{2} + {\rho_{I}^{2}I_{x}^{2}} - {2I_{d}\rho_{I}I_{x}}}{2\sigma_{I}^{2}} + \frac{Q_{d}^{2} + {\rho_{Q}^{2}Q_{x}^{2}} - {2Q_{d}\rho_{I}Q_{x}}}{2\sigma_{Q}^{2}}} \right\rbrack} - {\underset{x \in {C{({1,i})}}}{in}\left\lbrack {\frac{I_{d}^{2} + {\rho_{I}^{2}I_{x}^{2}} - {2I_{d}\rho_{I}I_{x}}}{2\sigma_{I}^{2}} + \frac{Q_{d}^{2} + {\rho_{Q}^{2}Q_{x}^{2}} - {2Q_{d}\rho_{I}Q_{x}}}{2\sigma_{Q}^{2}}} \right\rbrack}}$

Each component is minimised when their respective:

$\frac{I_{d}\rho_{I}I_{x}}{2\sigma_{I}^{2}} + \frac{Q_{d}\rho_{Q}Q_{x}}{2\sigma_{Q}^{2}}$

are maximised. Thus, another strategy is to compute and maximise onlythese terms for each set C(k,:), and to compute the full LLR only forthe constellation points that produce these maxima.

1-D Demapper

The Demapper can be significantly simplified into 2×1-D Demappers butwith some loss in performance. In any uniform QAM half the bits for eachconstellation point are completely determined by one of the axis, asillustrated for example in FIG. 28. For the example shown in FIG. 28,which corresponds to the example of 16QAM we can see that {b₀, b₂} aredetermined only by the real axis whilst {b₁, b₃} are determined only bythe imaginary axis. The mapping is tabulated as in the table below:

Bit Real Imaginary Position Value Amplitude Amplitude 0 0 +1, +3 1 −1,−3 1 0 +1, +3 1 −1, −3 2 0 −3, +3 1 −1, +1 3 0 −3, +3 1 −1, +1 16QAMbits versus axis amplitudes

This means that the soft decision values for bits {b₁, b₃} can bederived only from the LLR of amplitudes of the imaginary axis whilstthose for {b₀, b₂} can be derived only from the LLR of amplitudes of thereal axis. Thus the LLR calculation is simplified to the equation below,which would provide a 1D demapper:

${P\left( x \middle| r \right)} = {\frac{1}{\sigma_{I}\sqrt{2\pi}}{\exp\left( {- \frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}}} \right)}\mspace{14mu}{or}}$${P\left( x \middle| r \right)} = {\frac{1}{\sigma_{Q}\sqrt{2\pi}}{\exp\left( {- \frac{{{Q_{d} - {\rho_{Q}Q_{x}}}}^{2}}{2\sigma_{Q}^{2}}} \right)}}$

This equation is difference dependent on whether the LLR for either {b₀,b₂} or {b₁, b₃} are being calculated respectively. Thus taking {b₀, b₂}:

${L\left( b_{i} \right)} = {\ln\left\lbrack \frac{\sum\limits_{x \in {A{({1,i})}}}{\exp\left( {- \frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}}} \right)}}{\sum\limits_{x \in {A{({0,i})}}}{\exp\left( {- \frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}}} \right)}} \right\rbrack}$

where A(k,:) is the set of I-axis amplitudes for which b_(i)=k. Thenafter the max-log approximation:

${L\left( b_{i} \right)} = {{\underset{x \in {A{({0,i})}}}{in}\left\lbrack \frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}} \right\rbrack} - {\underset{x \in {A{({1,i})}}}{in}\left\lbrack \frac{{{I_{d} - {\rho_{I}I_{x}}}}^{2}}{2\sigma_{I}^{2}} \right\rbrack}}$

As before, each component is minimised when:

$\frac{I_{d}\rho_{I}I_{x}}{2\sigma_{I}^{2}}$

is maximised. The same analysis can be done for the imaginary axis.

Thus in an alternative embodiment the local equaliser/demapper 402 canbe arranged to perform either the 2D-demapper calculation explainedabove, or the 1-D demapper which typically is less complex than the 2-Ddemapper particularly as the number of constellation points in each ofthe sets A(0,:) and A(1,:) is rather low, which for the example of 16QAMrequires only two elements in each case as seen in Table 3. The max-logapproximation is thus often dispensed with for lower order modulationschemes and the LLR derived using piece-wise linear approximations ofthe exponential functions in the 1D-demapper equation since the numberof terms to sum is limited.

Second National Equaliser/De-mapper

As explained above in some example embodiments the estimate of the localsignal D′(z) can be used to re-constitute the received signal with theeffect that an improved estimate of the national signal S(z) can then begenerated from that re-constituted signal. As illustrated for the secondexample of an OFDM detector 150 shown in FIG. 25, a second nationalequaliser/demapper 400 is used to generate this improved estimate of thenational signal S(z). The second national equaliser/demapper 400 doesnot use a divide by zero calculation and bit slicing as is performed bythe first national equaliser 292, but like the local equaliser/demapper402, generates a log likelihood ratio for each modulation symbol orcell. As shown in FIG. 29, the second national equaliser/de-mapperincludes a subtraction circuit 420, which receives on a first input andoutput from a multiplier 422. The multiplier then feds a national signalequaliser/de-mapper 424 which operates in a similar manner to the localequaliser/de-mapper 404 to calculate for each modulation symbol or OFDMcell a LLR.

In FIG. 29, the multiplier 422 receives the estimate of the local signalD′(z) on a connecting channel 406 from the local equaliser/demapper 402and the estimate of the local channel H′l(z) on a connecting channel 408and combines the two to form an estimate of the local signal as receivedfrom the local channel. This resulting signal is fed to the first inputof the subtraction circuit 420, which receives on a second input theoriginally received signal R(z) from the FFT processor 290 as shown inFIG. 25. The output of the subtraction circuit 420 therefore formsS(z)H(z) according to the equation:R(z)−D′(z)H _(l)′(z)=S(z)H(z)

As shown in FIG. 29 the national signal demapper 424 operatessubstantially as explained above for the local equaliser/de-mapper togenerate a LLR value for each of the modulation symbols or cells of thereceived OFDM symbol for the national signal using H(z) received fromthe first national equaliser 292 as the channel transfer function toform a refined estimate of the national signal S″(z) at an output 160.

Summary of Operation

In summary the operation of the receiver shown in FIGS. 24a and 25 torecover the local data from the local service insertion symbols isillustrated at a general level by a flow diagram shown in FIG. 30 whichis summarised as follows:

S2: An estimate of the national broadcast symbols Ŝ(z) is formed byregarding the term

$\frac{H_{l}(z)}{\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}{D(z)}$as noise and slicing the recovered signal about the real and imaginaryplane to form an estimate of the national broadcast data.

S4: An estimate of the combined channel which is the transmittingchannel from the nation broadcast base station and the local serviceinsertion base station is formed using the main pilot sub-carriers Ps tocalculate an estimate of a term representing the regenerated nationalbroadcast signal convolved with the combined national broadcast andlocal service insertion channels Ŝ(z)+[H_(n)(z)+H_(l)(z)].

S6: An estimate of the local service insertion symbols convolved withthe local channel is formed by subtracting the generated term from stepS4 from the received signalR(z)(D(z)H_(l)(z)≈R(z)−Ŝ(z)[H_(n)(z)+H_(l)(z)]).

S8: An estimate of the channel through which the local service insertionhas passed from the base station to the receiver Ĥ_(l)(z) is determinedusing the local service insertion pilots.

S10: The local service insertion data is then estimated from the symbolsproduced by dividing the recovered term by the estimate of the localchannel

${\overset{\sim}{D}(z)} \approx {\frac{{R(z)} - {{\hat{S}(z)}\left\lbrack {{H_{n}(z)} + {H_{l}(z)}} \right\rbrack}}{{\hat{H}}_{l}(z)}.}$

Various modifications maybe made to the present invention describedabove without departing from the scope of the present invention asdefined in the appended claims. For example, other modulation schemescould be used other than those described above, with appropriateadjustments being made to the receiver. Furthermore, the demodulationprocess can be iterated as described above for a number of times toimprove the received symbol estimates. Furthermore, the receiver couldbe used in various systems, which utilise OFDM modulation other thanthose defined according to the DVB-NGH standards.

The invention claimed is:
 1. A receiver, comprising: circuitryconfigured to receive and recover an Orthogonal Frequency DivisionMultiplexed (OFDM) signal comprising OFDM symbols representing a firstphysical layer pipe in the presence of a second physical layer pipemodulated onto subcarriers of an OFDM symbol, the first physical layerpipe comprising first modulation symbols that are more easily recoveredthan second modulation symbols of the second physical layer pipe due tothe first modulation symbols requiring a lower signal to noise ratio forrecovery than the second modulation symbols; and OFDM detector circuitryconfigured to recover the first modulation symbols from the firstphysical layer pipe; regenerate an estimate of a component of the firstphysical layer pipe in the received OFDM signal; subtract the estimatefrom the OFDM symbol; and recover the second modulation symbols from thesecond physical layer pipe, wherein the received OFDM signal includesOFDM symbols which include both of the first physical layer pipe and thesecond physical layer pipe, and wherein the received OFDM signalcomprises frames of OFDM symbols.
 2. The receiver according to claim 1,wherein the second physical layer pipe carries local service datasymbols.
 3. The receiver according to claim 1, wherein the firstphysical layer pipe carries national service data symbols.
 4. Thereceiver according to claim 1, wherein the first modulation symbols ofthe first physical layer pipe are recoverable from the OFDM signal bydecoding a first modulation scheme and the second modulation symbols ofthe second physical layer pipe are recoverable by decoding a second,different modulation scheme.
 5. The receiver according to claim 4,wherein the second modulation scheme is a higher order modulation schemethan the first modulation scheme.
 6. The receiver according to claim 5wherein the first modulation scheme is a 16 QAM modulation Scheme. 7.The receiver according to claim 5 wherein the second modulation schemeis a 64QAM modulation scheme.
 8. The receiver according to claim 1wherein the first physical layer pipe is recoverable in the presence ofnoise caused by the second physical layer pipe.
 9. The receiveraccording to claim 8, wherein the signal to noise ratio of the firstphysical layer pipe is greater than that of the second physical layerpipe.
 10. The receiver according to claim 1 being further configured touse the recovered data from the second physical layer pipe to refinerecovering of data from the first physical layer pipe.
 11. The receiveraccording to claim 1 wherein the received OFDM signal is a combinationof a signal representing the first physical layer pipe and a signalrepresenting the second physical layer pipe.
 12. A television receivercomprising tuner and the receiver according to claim
 1. 13. A mobilereceiver comprising a tuner and the receiver according to claim
 1. 14. Amethod comprising: receiving and recovering an Orthogonal FrequencyDivision Multiplexed (OFDM) signal comprising OFDM symbols representinga first physical layer pipe in the presence of a second physical layerpipe modulated onto subcarriers of an OFDM symbol, the first physicallayer pipe comprising first modulation symbols that are more easilyrecovered than second modulation symbols of the second physical layerpipe due to the first modulation symbols requiring a lower signal tonoise ratio for recovery than the second modulation symbols; recoveringthe first modulation symbols from the first physical layer pipe;regenerating, using circuitry, an estimate of a component of the firstphysical layer pipe in the received OFDM signal; subtracting theestimate from the OFDM symbol; and recovering the second modulationsymbols from the second physical layer pipe, wherein the received OFDMsignal includes OFDM symbols which include both of the first physicallayer pipe and the second physical layer pipe, and wherein the receivedOFDM signal comprises frames of OFDM symbols.
 15. The method accordingto claim 14, wherein the second physical layer pipe carries localservice data symbols.
 16. The method according to claim 14, wherein thefirst physical layer pipe carries national service data symbols.
 17. Themethod according to claim 14, wherein the first modulation symbols ofthe first physical layer pipe are recoverable from the OFDM signal bydecoding a first modulation scheme and the second modulation symbols ofthe second physical layer pipe are recoverable by decoding a second,different modulation scheme.
 18. The method according to claim 17,wherein the second modulation scheme is a higher order modulation schemethan the first modulation scheme.
 19. The method according to claim 18wherein the first modulation scheme is a 16 QAM modulation Scheme. 20.The method according to claim 19 wherein the second modulation scheme isa 64QAM modulation scheme.
 21. The method according to claim 14comprising recovering the first physical layer pipe in the presence ofnoise caused by the second physical layer pipe.
 22. The method accordingto claim 21, wherein the signal to noise ratio of the first physicallayer pipe is greater than that of the second physical layer pipe. 23.The method according to claim 14 comprising using the recovered datafrom the second physical layer pipe to refine recovering of data fromthe first physical layer pipe.
 24. The method according to claim 14wherein the received OFDM signal is a combination of a signalrepresenting the first physical layer pipe and a signal representing thesecond physical layer pipe.
 25. A non-transitory storage mediumcomprising computer readable instructions which when executed oncomputer perform a method, the method comprising: receiving andrecovering an Orthogonal Frequency Division Multiplexed (OFDM) signalcomprising OFDM symbols representing a first physical layer pipe in thepresence of a second physical layer pipe modulated onto subcarriers ofan OFDM symbol, the first physical layer pipe comprising firstmodulation symbols that are more easily recovered than second modulationsymbols of the second physical layer pipe due to the first modulationsymbols requiring a lower signal to noise ratio for recovery than thesecond modulation symbols; recovering the first modulation symbols fromthe first physical layer pipe; regenerating an estimate of a componentof the first physical layer pipe in the received OFDM signal;subtracting the estimate from the OFDM symbol; and recovering the secondmodulation symbols from the second physical layer pipe, wherein thereceived OFDM signal includes OFDM symbols which include both of thefirst physical layer pipe and the second physical layer pipe, andwherein the received OFDM signal comprises frames of OFDM symbols.